Objective. To determine if folinic acid supplementation during methotrexate (MTX) therapy for rheumatoid arthritis (RA) reduces both urinary 5-aminoimidazole-4-carboxamide (AICA) and urinary adenosine excretion more than does folic acid supplementation. AICA and adenosine are markers for MTX interference with purine metabolism. Methods. Forty patients with RA who received MTX for 6 weeks were randomized to receive either daily folic acid or folinic acid supplements during an additional week of MTX therapy. Colorimetric and radioimmunocompetition assays were used to measure 24-hour urinary AICA and adenosine excretion levels, respectively. Results. At the end of 6 weeks, 24-hour urinary levels of AICA, but not adenosine, were elevated as compared with baseline levels (i.e., prior to MTX therapy). Folinic acid, but not folic acid, supplementation normalized urinary AICA levels during MTX therapy. Relatively high urinary levels of AICA were correlated with reduced disease activity. No similar correlations were seen with urinary adenosine levels. Conclusion. The blockade of purine nucleotide biosynthesis by MTX at the AICA ribonucleotide transformylase-catalyzed step may be related to the efficacy of MTX, and this blockade is effectively relieved by folinic acid, but not by folk acid, supplementation.